Spiral type cryogenic freezers that are currently in commercial use are as near as possible to being an isothermal chamber. The freezers such as shown in U.S. Pat. Nos. 4,866,946, 4,739,623 and 4,612,780 have a spiral type conveyor inside of an insulated box shaped chamber with temperatures maintained at or near the desired refrigeration temperature. For example, such freezers can use liquid nitrogen, liquid carbon dioxide or mechanically refrigerated air to achieve the freezing process.
One of the problems with the spiral type freezer is the fact that the box like insulated chamber results in heat transfer coefficients that are lower than those normally found with in-line tunnel freezers and therefore the spiral type freezer is by and large an inefficient device. In-line tunnel freezers can be lengthy for high production rates although the length does not effect the process efficiency.
Prior art spiral freezer have focused on trying to efficiently use the refrigeration inside of the insulated container by employing circulating fans, baffles and pressurized chambers at the inlet or outlet of the freezing chamber in order to avoid the problems of low efficiency.
One of the problems with prior art spiral type freezers is that the chamber or insulated box that contains the spiral conveyor is of necessity larger than the spiral disposed inside so that workers can enter the chamber during periods of non-use to clean the interior of the chamber and the conveyor. This is especially necessary if the freezer is used for food products.
Attempts have been made to miniaturize or reduce the size of the freezer, one such freezer being offered for sale by Fuji Tetsumo of Japan. However this freezer has the entire refrigeration apparatus disposed inside the container and thus increases the problem with trying to maintain cleanliness of the freezer.